PB138 — XML Schema

Basic sources of information

XML Schema Specification
Using W3C XML Schema Tutorial brief
XML Schema Tutorial at W3Schools
More comprehensive complete tutorial at xfront.com
How to add XML Schema support to Netbeans IDE available at Geertjan’s Blog
Try this XML Schema online validator or
Similar but more general Validome validator (not only XML schema) alternativelly available in web archive.

Motivation

Stronger tool for XML data model specification than DTD, it allows:

Separate type (e.g. element type) from its occurrence (i.e. element with particular name)
More primitive data types.
Allows to use namespaces.
Allows to specify content model (elements) more accurate way.
Allows new type inheritance.
Allows modular schema design and schema reuse.
XML Schema has an XML syntax.

XML Schema Definition Header

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
     .../...
</xs:schema>

Assignment of type to element with the given name

<xs:element name="element_name">
 <!-- here comes the type definition --
 placed either right here (so called "local")
 or as a referenced one (so called "global") -->
</xs:element>

short type reference:

<xs:element name="element_name" type="element_type"/>

Simple Type Definition

Does not contain any child elements. Can be used like either element or attribute type.
Possible to define using an existing type restriction

<xs:simpleType name="TypeName">
   <xs:restriction base="BaseTypeName"> ... </xs:restriction>
</xs:simpleType>

Simple type definition (Example 1)

Content length restriction

<xs:simpleType name="nameType">
   <xs:restriction base="xs:string"> <xs:maxLength value="32"/>
</xs:restriction> </xs:simpleType>

Simple type definition (Example 2)

Content restriction using a regular expression

<xs:simpleType name="isbnType">
  <xs:restriction base="xs:string"> <xs:pattern value="[0-9]{10}"/> </xs:restriction>
</xs:simpleType>

Simple types — `union`

Approximately correspond to C "union" concept.
Result is a simple type.
Base type and values enumeration can be merged.

 <xs:simpleType name="isbnType">
  <xs:union>
    <xs:simpleType>
     <xs:restriction base="xs:string">
      <xs:pattern value="[0-9]{10}"/>
     </xs:restriction>
    </xs:simpleType>
    <xs:simpleType>
     <xs:restriction base="xs:NMTOKEN">
      <xs:enumeration value="TBD"/>
      <xs:enumeration value="NA"/>
     </xs:restriction>
    </xs:simpleType>
  </xs:union>
 </xs:simpleType>

Simple types - values enumeration

Type can be defined as a values list separated by white-spaces.
The number of elements list limitation can used as a next derivation type.

<xs:simpleType name="isbnTypes">
  <xs:list itemType="isbnType"/>
</xs:simpleType>
<xs:simpleType name="isbnTypes10">
  <xs:restriction base="isbnTypes">
    <xs:minLength value="1"/>
    <xs:maxLength value="10"/>
  </xs:restriction>
</xs:simpleType>

Complex type definition

<xs:complexType name="TypeName">
  <xs:sequence>
    <xs:element ...> ...
       <xs:attribute ...>
    </xs:element>
  </xs:sequence>
</xs:complexType>

<xs:choice> and <xs:all> can be used instead of sequence.

Complex type definition — element groups

The group element can be used to define complex type.
Group of elements:

<xs:group name="GroupName">
  <xs:sequence>
       <xs:element ... /> ...
  </xs:sequence>
</xs:group>

<xs:choice> and <xs:all> can be used instead of sequence.

Complex type definition — attribute groups

Attribute group:

 <xs:attributeGroup name="AttributesGroupName">
       <xs:attribute ... use="required"/>
       ...
 </xs:attributeGroup>

The mandatory occurrence may be specified (use="required").

Groups usage

Example of elements/attributes groups use:

 <xs:complexType name="bookType">
  <xs:sequence>
   <xs:group ref="mainBookElements"/>
   <xs:element name="character" type="characterType" minOccurs="0" maxOccurs="unbounded"/>
  </xs:sequence>
  <xs:attributeGroup ref="bookAttributes"/>
 </xs:complexType>

Compositor `sequence`

Defines occurrence of elements in the predefined order.

 <xs:element name="element_name">
  <xs:complexType>
       <xs:sequence>
        .../...
       </xs:sequence>
       .../...
  </xs:complexType>
 </xs:element>

sequence is a content model that allows occurrence of the defined sequence of child elements.
xs prefix is (as usually) bound to the NS with URL http://www.w3.org/2001/XMLSchema
Either <xs:choice> or <xs:all> can be used instead of <xs:sequence>.

Compositor `choice`

Defines the occurrence of only one of the specified child elements or groups of elements.

 <xs:element name="element_name">
  <xs:complexType>
    <xs:choice>
      .../...
    </xs:choice>
    .../...
  </xs:complexType>
 </xs:element>

Compositor `all`

Defines occurrence of child elements without definition of their order.
May appear on the definition top level only.
The cardinality of child elements can be one at most.

 <xs:complexType name="bookType">
  <xs:all>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="author" type="xs:string"/>
    <xs:element name="character"type="characterType" minOccurs="0" maxOccurs="1"/>
  </xs:all>
  <xs:attribute name="isbn" type="isbnType" use="required"/>
 </xs:complexType>

Element simple content

 <xs:element name="book">
  <xs:complexType>
   <xs:simpleContent>
     <xs:extension base="xs:string">
      <xs:attribute name="isbn" type="isbnType"/>
     </xs:extension>
   </xs:simpleContent>
  </xs:complexType>
 </xs:element>

Mixed element content

The text content (textual child nodes) can not be validated.
The child elements can be validated.

 <xs:element name="book">
  <xs:complexType mixed="true">
   <xs:all>
    <xs:element name="title" type="xs:string"/>
    <xs:element name="author" type="xs:string"/>
   </xs:all>
   <xs:attribute name="isbn" type="xs:string"/>
  </xs:complexType>
 </xs:element>

Further options

Possibility to specify integrity limitations:
- value is unique — xs:unique
- value is a key — xs:key
- value is a key reference — xs:keyref

Schema annotation

Annotation is a human-readable note (comment) of a schema or its part.
It may contain the processing information, see example xs:appinfo as well.
Next content is not specified (limited), see example (bind, class, …)

 <xs:annotation>
  <xs:documentation xml:lang="en">Top level element.</xs:documentation>
  <xs:documentation xml:lang="fr">Element racine.</xs:documentation>
  <xs:appinfo source="http://example.com/foo/">
    <bind xmlns="http://example.com/bar/">
     <class name="Book"/>
    </bind>
  </xs:appinfo>
 </xs:annotation>

Schema definition reuse

Direct:

 <xs:include schemaLocation="character.xsd"/>

With redefinition:

<xs:redefine schemaLocation="character12.xsd">
  <xs:simpleType name="nameType">
   <xs:restriction base="xs:string">
    <xs:maxLength value="40"/>
   </xs:restriction>
  </xs:simpleType>
</xs:redefine>

Abstract and final types

abstract

Type can not be instantiated.

Can be used for inheritance derivation only.

final

Type can not be extended/derived by inheritance.

Namespaces in XML Schema

<xs:schema targetNamespace="http://example.org/ns/books/"
       xmlns:xs="http://www.w3.org/2001/XMLSchema"
       xmlns:bk="http://example.org/ns/books/" elementFormDefault="qualified"
       attributeFormDefault="unqualified">
   .../...
</xs:schema>

Unspecified elements and attributes

XML Schema allows to use some elements that are not known prior to its use.

<xs:complexType name="descType" mixed="true">
  <xs:sequence>
   <xs:any namespace="http://www.w3.org/1999/xhtml"
           processContents="skip" minOccurs="0" maxOccurs="unbounded"/>
  </xs:sequence>
</xs:complexType>

Use xs:anyAttribute for attributes.

Schema definition reference

<book isbn="0836217462"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="file:library.xsd">
<book isbn="0836217462" xmlns="http://example.org/ns/books/"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="file:library.xsd">